题目内容
— Do you know ?
— In the 6th century.
— In the 6th century.
[ ]
A. who is the abacus invented
B. when the abacus is invented
C. when was the abacus invented
D. when the abacus was invented
B. when the abacus is invented
C. when was the abacus invented
D. when the abacus was invented
D
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